1. Field of the Invention
This invention relates to a television system converter which converts a high-definition television signal into an NTSC signal of the present television system.
2. Description of the Related Art
A high-definition television system, which differs from the present television system in terms of ratio, number of scanning lines, and frame frequency has been developed. For example, there is a MUSA (Multiple Sub-Nyquist Sampling-Encoding) system. The MUSE system is disclosed in INTERNATIONAL BROADCASTING SYMPOSIUM ON RADIO AND TELEVISION ENGINEERING 1985 SESSION 2-(7) "Transmission of HDTV by MUSE system" by Yuichi Ninomiya et al.
The transmission of MUSE signals by broadcast satellite should now be in the stage of actual transmission for practical use. To receive MUSE signals and see them as an image, a MUSE decoder and a MUSE display are required. However, MUSE decoder and display technology is not commonly available. Therefore, there is a need for a device which can receive MUSE signals and convert them for use by an NTSC system, and subsequently display the converted signal on a NTSC system.
In converting a MUSE signal into an NTSC signal, it should be notice that the field frequency of the MUSE signal is 60 Hz and that of the NTSC signal is 59.94 Hz.
Another point to notice is that a frequency of the horizontal scanning line of the MUSE signal is 30.times.525=15.75 KHz and that of the NTSC signal is 59.94.div.2.times.525=15.734 KHz. Also, a frequency of color sub-carrier fsc is defined to a 1/2 multiple of the integral of one period of the horizontal scanning line, that is, 455/2.
If the television signal having field frequency of 60.00 Hz is used similar to NTSC system, color sub-carrier fsc' is 60.00/59.94.times. fsc. The color sub-carrier fsc' has differences of about 1000 ppm as compared with the sub-carrier fsc of the NTSC signal.
In general, the frequency draw-in range of the color sub-carrier in a television receiver is about fsc .+-.400 Hz (=100 mmp). If there are about 1000 ppm differences, such a frequency cannot draw in the color sub-carrier. Therefore, if the color sub-carrier having such a difference is input into a color signal processor, a color killer circuit is operated, so that an image becomes monochrome.
To overcome this problem, if a normal color sub-carrier fsc is used, the relationship of an interleave between a luminance signal such as a reference NTSC signal and the color sub-carrier cannot be completely maintained. As a result, interruptions such as a cross color and a cross luminance are frequently generated in a receiver.